1. Field of the Invention
This invention relates in general to subsea well production, and in particular to a subsea well that has vertical production passages and tubing annulus communication other than through the tubing hanger.
2. Background of the Invention
A subsea well that is capable of producing oil or gas will have an outer or low pressure wellhead housing secured to a string of conductor pipe that extends some short depth into the well. An inner or high pressure wellhead housing lands in the outer wellhead housing. The tubular wellhead member is secured to an outer string of casing, which extends through the conductor pipe to a deeper depth into the well. Depending on the particular conditions of the geological strata above the target zone (typically, either an oil or gas producing zone or a fluid injection zone), one or more additional casing strings will extend through the outer string of casing to increasing depths in the well until the well is to the final depth.
The last string of casing extends into the well to the final depth, this being the production casing. The strings of casing between the first casing and the production casing are intermediate casing strings. When each string of casing is hung in the wellhead assembly, a cement slurry is flowed through the inside of the casing, out of the bottom of the casing, and back up the outside of the casing to a predetermined point. A tubing hanger typically connects to the wellhead assembly. When a string of production tubing is lowered into the well and supported by the tubing hanger, a tubing annulus is defined between the outer surface of the production tubing and the innermost or production casing. At a lower portion of the production tubing, a seal system or packer is typically connected to the outer surface of the production tubing and the inner surface of the production casing. After the production casing is perforated, well fluids enter the well through the perforations to communicate up the interior of the production tubing to the wellhead.
Sometimes it is desirous for a heavy fluid or “kill” fluid to be pumped either through the tubing or from the tubing annulus past the packer so that the operator can stop production from the well before removing the string of production tubing. In situations where the heavy fluid is pumped down the tubing annulus, the packer may be released for allowing fluid to flow below the packer to the interior of the tubing.
Additionally, operators desire a means of communicating and monitoring pressure of producing wells in the production tubing annulus. Normally there should be no pressure in the production tubing annulus because the annular space is sealed with the packer. If pressure increased within the production tubing annulus, it would indicate that a leak exists in one of the strings of casing or in the tubing. The leak could be from several places. Regardless of where the leak is coming from, pressure build up in the production tubing annulus could collapse a portion of the production tubing, compromising the structural and pressure integrity of the well. For this reason, operators typically monitor the pressure in the production tubing annulus between the production casing and the production tubing.
In one type of wellhead assembly, the tubing hanger has an offset passage through it for communicating with the tubing annulus. In this type, the tubing hanger lands in the wellhead housing or in a tubing spool above the wellhead housing. In small diameter tubing hangers, there may not be enough space for the tubing annulus passage. In another type, the tubing hanger lands in a production tree mounted on the wellhead housing. A bypass passage extends through the tree around the tubing hanger for communicating with the tubing annulus.